The present invention relates generally to two-stage telescopic hydraulic cylinders for use with car carriers, and more particularly to a system of channeling hydraulic fluids through the external cylinder of a multiple stage cylinder so as to accomplish distension and retraction of both cylinder pistons without the use of a third hydraulic line and to eliminate the presence of the third hydraulic line from the car carrier.
Two-stage telescopic hydraulic cylinders usually comprise an outer housing and two ram elements, with each ram element including a piston. The ram elements operate in a telescoping manner relative to one another and relative to the outer housing. The first ram element moves in and out of the housing and the second ram element moves in and out of the first ram element. The purpose of using the two-stage cylinder instead of a single stage cylinder is to achieve a long arm movement from a relatively short cylinder length.
To distend both ram elements of a typical prior art two stage telescopic cylinder, hydraulic fluid is applied to both pistons from the base end of the outer cylinder body or housing. The fluid is delivered by hydraulic lines or tubing from a hydraulic fluid supply to a single entry port in the base of the cylinder housing and channeled internally to contact and apply pressure against the rear faces of both of the two pistons.
To retract the ram elements, fluid must be applied to the opposite, outer faces of the two pistons. Since, in their distended positions, the pistons are located at the outer end of the housing and at the outer end of the first ram element, it is necessary to apply hydraulic fluid to the outer ends of both the housing and the first ram element. Typical prior art two-stage cylinders accomplished the application of hydraulic fluid to the outer surfaces of the two pistons by placing a fluid entry port in both locations, one at the outer end of the housing and one at the outer end of the first ram element. These two fluid entry ports, which represent the second and third ports of the two-stage hydraulic cylinder, are connected to the hydraulic fluid supply by flexible hydraulic lines or tubing, extending along the exterior of the cylinder.
The external hydraulic line connected to the port at the end of the first ram element must be long enough to reach the port when the ram element is both fully distended and fully retracted. These hydraulic lines are sometimes wound around the external surfaces of the cylinder like a coil spring so as to progressively expand and retract with the expansion and retraction of the cylinder, but the coiled configuration of the hydraulic lines requires additional length of tubing. Another hydraulic line arrangement provided for two-stage cylinders includes a coil tension spring connected to the hydraulic line and to some stationary object to bias the hydraulic line to a retracted, out of the way, position, but this requires the additional spring element and a relatively long hydraulic line. In all of the two-stage telescopic hydraulic cylinders which require a hydraulic line to move with the movement of the ram elements, the hydraulic lines are exposed and therefore are subject to damage by abrasion and deterioration and are sometimes cut or ruptured by tangling with other objects.
One use of two-stage telescopic hydraulic cylinders is with car carrying trailers, in which a trailer carries trucks and cars in two or more levels on public roads. The upper platforms of the typical trailer which carry the upper level of cars must be lowered to receive the cars and raised a substantial distance so that cars can be loaded on the lower level of the trailer. It is desirable to use the multiple-stage hydraulic cylinders on the trailers to include the desired long reach feature of the cylinders, but the presence of the cumbersome external hydraulic lines that must move with the ram elements is a major problem because the lines get in the way of the trailer operator, are sometimes caught in the elements of the trailer or in the vehicles moving on or off the trailer, and are subject to abrasion and deterioration during use of the trailer.